The present invention relates to a method for measuring the range between a transmitter or a receiver and a terminal in a wireless communication to calculate the position of the terminal from the range measurement result. More particularly, the present invention relates to a method for calculating the position of a terminal using a delay time from a wireless base station to a terminal (or from a terminal to a wireless base station).
In a mobile communication system, some technologies for calculating the position of a terminal with the use of the signals from a base station have been proposed. For example, in JP-A-7-181242 laid-open on Jul. 21st, 1995, a technology for measuring the position of a terminal is proposed for use in a Code Division Multiple Access (CDMA) system. The proposed technology is that base station positions and the propagation time of a signal transmitted from each of the base stations to a terminal are used to calculate the differences in the PN code transmission times from the base stations. These time differences are then used to calculate the position of the terminal.
In calculating the position of a terminal from the multiple range measurement results, the position of the terminal is usually calculated using the least square method on the assumption that position calculation errors are distributed according to the Gaussian distribution. In addition, to calculate the terminal position more precisely, another position calculation method is sometimes used in which the range is measured multiple times for calculating the terminal position, the multiple calculated positions are averaged, and the center of gravity of the multiple positions are used as the result of measurements.
In the conventional position calculation methods described above, the least square method is used to calculate the position of a terminal assuming that position data collected by range measurements is distributed according to the Gaussian distribution. This method is based on the assumption that positive and negative range measurement errors occur evenly, that is, lags and leads occur with an equal probability in the signal from the terminal to the base station.
Range measurement errors are generated primarily by two causes: noises and multi-path reflections caused by signal reflections. In general, noises have an effect when the S/N ratio of received signals is low. When noises are major causes of errors (with little or no multi-path reflection), the least square provides gives the most likely solution.
On the other hand, multi-path reflections may have an effect regardless of the S/N ratio, and this effect may be serious even when the signal strength is high (good S/N ratio). The delay profiles of received signals affected by multi-path reflections are not always in the Gaussian distribution, but sometimes in an asymmetric distribution of positive and negative values. Therefore, when the measurement is made under good S/N ratio conditions but with some multi-path reflections, the position calculation made by the least square method does not provide the most likely solution.
Another well-known method for increasing the precision is to take multiple measurement and to calculate the position with the measurement results to give the center of gravity of calculated multiple points (arithmetic mean of multiple coordinates) as the measurement result of the points. However, depending upon how measurement positions are distributed, this arithmetic mean does not always provide a correct position.
It is an object of the present invention to provide a position calculation method that gives a correct position even when measurement errors are not distributed symmetrically for positive and negative values.
According to one aspect of the present invention, there is provided a position calculation method wherein a terminal receives signals from a plurality of antennas located at known and different positions and from a base antenna, or the plurality of antennas located at known and different positions and the base antenna receive signals from the terminal, for calculating a position of the terminal with wireless signal delay times. The method comprising:
a first procedure for calculating, for each of the plurality of antennas, a first range that is a difference between a range from the terminal to each of the plurality of antennas, the range being calculated based on a wireless signal delay time, and a range from the terminal to the base antenna, the range being calculated based on a wireless signal delay time;
a second procedure for calculating, for each of the plurality of antennas, a second range that is a difference between a range from an assumed terminal position for calculation purposes to each of the plurality of antennas and a range from the assumed terminal position for calculation purposes to the base antenna;
a third procedure for calculating, for each of the plurality of antennas, an error that is a difference between the first range and the second range;
a fourth procedure for calculating, assuming that the errors have a distribution asymmetric for positive and negative values, a likelihood of the assumed terminal position using the distribution and the errors each calculated for each of the plurality of antennas; and
a fifth procedure for repeating the second to fourth procedures to obtain a point where the likelihood is maximized and for determining an obtained most likely solution as the position of the terminal.
According to another aspect of the present invention, there is provided a position calculation apparatus comprising a receiver that receives signals from a plurality of antennas located at known and different positions and from a base antenna, a delay profile analysis apparatus that performs delay profile analysis for the signals received by the receiver, and a calculation apparatus that uses wireless signal delay times included in delay profile analysis results, produced by the delay profile analysis apparatus, to calculate a position of the position calculation apparatus, wherein
the calculation apparatus comprises:
first range calculation means for calculating, for each of the plurality of antennas, a first range that is a difference between a range from the terminal to each of the plurality of antennas, the range being calculated based on a wireless signal delay time, and a range from the terminal to the base antenna, the range being calculated based on a wireless signal delay time;
second range calculation means for calculating, for each of the plurality of antennas, a second range that is a difference between a range from an assumed terminal position for calculation purposes to each of the plurality of antennas and a range from the assumed terminal position for calculation purposes to the base antenna;
error calculation means for calculating, for each of the plurality of antennas, an error that is a difference between the first range and the second range;
likelihood calculation means for calculating, assuming that the errors calculated by the error calculation means have a distribution asymmetric for positive and negative values, a likelihood of the assumed terminal position using the distribution and the errors each calculated for each of the plurality of antennas; and
terminal position search means for obtaining a point where the likelihood of the assumed terminal position is maximized and for determining an obtained most likely solution as the position of the terminal.
A method in an embodiment according to the present invention uses multiple measurement results to calculate the most likely solution with the probability distribution function taken into consideration. Therefore, when calculating the position of a terminal using range measurement results based on wireless signal delay times, the method calculates the position of the terminal precisely even when the signal strength is high and errors are generated primarily by multi-path reflections. That is, the method calculates the likelihood not with a Gaussian distribution but with a function that produces a distribution of actual range measurement errors, thus ensuring a most likely solution that is correct.